Solid electrolytic capacitors (e.g., tantalum capacitors) have been a major contributor to the miniaturization of electronic circuits and have made possible the application of such circuits in extreme environments. The anode of a typical solid electrolytic capacitor includes a porous anode body, with a lead wire extending beyond the anode body and connected to an anode termination of the capacitor. The anode can be formed by first pressing a tantalum powder into a pellet that is then sintered to create fused connections between individual powder particles. One problem with many conventional solid electrolytic capacitors is that upon sintering, such powders tend to shrink away and separate from an embedded lead wire. Typically, the lead wire is mostly cylindrically-shaped with a smooth surface, making connection to the tantalum particles of the pellet more difficult. In fact, on a straight lead wire, it is difficult to find any points of contact between the lead wire and the powder particles. More specifically, the anode body shrinks diametrically towards the midpoint of the anode body, whereas the inner diameter of the anode body (surrounding the lead wire) initially increases. This shrinkage greatly reduces the degree to which the lead wire is bonded to the particles of the anode body, thereby increasing equivalent series resistance (ESR) and decreasing electrical capabilities of the capacitor. The same is true laterally, where the anode body will shrink towards the midpoint and away from any contact with the lead wire.
While several efforts have been made to improve the connection between the anode body and anode lead wire, these efforts involve additional processing steps that can be disadvantageous from a manufacturing standpoint. For example, U.S. Patent Application Publication No. 2010/0274337 to Hintz, et al. describes the additional step of sintering a metal powder onto a portion of a lead wire to form a connection region at a temperature higher than that used for the de-oxidation sintering process. Additionally, U.S. Pat. No. 6,447,570 to Pozdeev-Freeman describes annealing at temperatures above the temperature used for de-oxidation to improve bonding of the anode body to the lead wire, which can contaminate and degrade the annealing furnace, or welding a lead wire to a pre-sintered anode body then de-oxidizing the composite structure.
As such, a need currently exists for an improved solid electrolytic capacitor that increases the points of contact between the lead wire and the anode body, thereby significantly improving electrical capabilities by achieving ultralow ESR levels.